1. Field of the Invention
The present invention relates to a power steering apparatus that applies a steering assist force to a steering mechanism by a hydraulic pressure that is generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering apparatuses are known which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor.
Drive control of the electric motor is performed, for example, on the basis of the steering angle of the steering wheel. That is, the steering angle is determined on the basis of an output of a steering angle sensor provided in association with the steering wheel, and the driving of the electric motor is controlled on the basis of the steering angle thus determined. More specifically, if the steering angle of the steering wheel is within a motor off range defined around a steering angle midpoint, steering assistance is considered to be unnecessary, so that the electric motor is stopped. On the other hand, if the steering angle of the steering wheel is out of the motor off range, the electric motor is actuated to generate a steering assist force.
Determination of the steering angle midpoint is achieved, for example, by sampling steering angle data outputted from the steering angle sensor and regarding the most frequent steering angle data as corresponding to the steering angle midpoint.
In the prior art described above, the electric motor is actuated in response to detection of a steering angle value falling out of the fixedly defined motor off range. If the motor off range is set wider, an entrapped feeling may occur due to a delay in the actuation of the electric motor, so that the steering feeling is deteriorated. Therefore, the motor off range is generally set narrower. However, this makes the actuation of the electric motor more sensitive to the operation of the steering wheel, resulting in a greater energy loss. That is, the electric motor may needlessly be actuated even when the steering is deflected due to traveling on a rough road or the like.